In mammals, the kidney is the organ primarily responsible for the control of body fluid volume and composition. An important part of this control is accomplished by variations in the rates of fluid exchange in the renal glomerular and peritubular capillaries. In recent years, both glomerular ultrafiltration and peritubular capillary fluid absorption have begun to be studied in vivo by means of single vessel micropuncture techniques developed largely in the principal investigator's laboratory. The availability of pressure, flow rate, and concentration measurements in these microvessels has provided us not only with essential data not heretofore available, but also with an incentive for developing a complementary theoretical treatment of renal transcapillary fluid exchange. During the current funding year, a one-dimensional model of glomerular ultrafiltration has been developed and shown to be useful in analyzing the dependence of single nephron glomerular filtration rate on glomerular plasma flow rate and mean transmembrane hydrostatic pressure difference. It is shown that changes in nephron glomerular filtration rate under most experimental conditions may be viewed almost entirely as the consequence of changes in glomerular plasma flow rate. The ultrafiltration coefficient is found to be independent of glomerular plasma flow rate under the conditions studied, and the glomerular effective hydraulic permeability is calculated to be one order of magnitude larger than previously reported by others. A similar one-dimensional treatment of peritubular capillary fluid absorption has also been developed during the current funding period. The results show that variations in peritubular capillary protein concentration are of prime importance in controlling proximal tubule fluid uptake, in close accord with recent experimental observations in rat and dog. The glomerular and peritubular capillary models taken together allow clarification of the mechanisms responsible for the well- known coupling between glomerular ultrafiltration and peritubular absorption and the manner in which the control of these transport processes relates to the maintenance of body fluid volume and composition.